GAPDH controls extracellular vesicle biogenesis and enhances the therapeutic potential of EV mediated siRNA delivery to the brain

Extracellular vesicles (EVs) are biological nanoparticles with important roles in intercellular communication, and potential as drug delivery vehicles. Here we demonstrate a role for the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in EV assembly and secretion. We observe high levels of GAPDH binding to the outer surface of EVs via a phosphatidylserine binding motif (G58), which promotes extensive EV clustering. Further studies in a Drosophila EV biogenesis model reveal that GAPDH is required for the normal generation of intraluminal vesicles in endosomal compartments, and promotes vesicle clustering. Fusion of the GAPDH-derived G58 peptide to dsRNA-binding motifs enables highly efficient loading of small interfering RNA (siRNA) onto the EV surface. Such vesicles efficiently deliver siRNA to multiple anatomical regions of the brain in a Huntington's disease mouse model after systemic injection, resulting in silencing of the huntingtin gene in different regions of the brain.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions.

Phosphodiesterase 2A2 regulates mitochondria clearance through Parkin-dependent mitophagy

Programmed degradation of mitochondria by mitophagy, an essential process to maintain mitochondrial homeostasis, is not completely understood. Here we uncover a regulatory process that controls mitophagy and involves the cAMP-degrading enzyme phosphodiesterase 2A2 (PDE2A2). We find that PDE2A2 is part of a mitochondrial signalosome at the mitochondrial inner membrane where it interacts with the mitochondrial contact site and organizing system (MICOS). As part of this compartmentalised signalling system PDE2A2 regulates PKA-mediated phosphorylation of the MICOS component MIC60, resulting in modulation of Parkin recruitment to the mitochondria and mitophagy. Inhibition of PDE2A2 is sufficient to regulate mitophagy in the absence of other triggers, highlighting the physiological relevance of PDE2A2 in this process. Pharmacological inhibition of PDE2 promotes a 'fat-burning' phenotype to retain thermogenic beige adipocytes, indicating that PDE2A2 may serve as a novel target with potential for developing therapies for metabolic disorders.

Full Rescue of F508del-CFTR Processing and Function by CFTR Modulators Can Be Achieved by Removal of Two Regulatory Regions

Cystic Fibrosis (CF) is caused by mutations in the CF Transmembrane conductance Regulator (CFTR), the only ATP-binding cassette (ABC) transporter functioning as a channel. Unique to CFTR is a regulatory domain which includes a highly conformationally dynamic region—the regulatory extension (RE). The first nucleotide-binding domain of CFTR contains another dynamic region—regulatory insertion (RI). Removal of RI rescues the trafficking defect of CFTR with F508del, the most common CF-causing mutation. Here we aimed to assess the impact of RE removal (with/without RI or genetic revertants) on F508del-CFTR trafficking and how CFTR modulator drugs VX-809/lumacaftor and VX-770/ivacaftor rescue these variants. We generated cell lines expressing ΔRE and ΔRI CFTR (with/without genetic revertants) and assessed CFTR expression, stability, plasma membrane levels, and channel activity. Our data demonstrated that ΔRI significantly enhanced rescue of F508del-CFTR by VX-809. While the presence of the RI seems to be precluding full rescue of F508del-CFTR processing by VX-809, this region appears essential to rescue its function by VX-770, suggesting some contradictory role in rescue of F508del-CFTR by these two modulators. This negative impact of RI removal on VX-770-stimulated currents on F508del-CFTR can be compensated by deletion of the RE which also leads to the stabilization of this mutant. Despite both regions being conformationally dynamic, RI precludes F508del-CFTR processing while RE affects mostly its stability and channel opening.

Targeting FRET-Based Reporters for cAMP and PKA Activity Using AKAP79

Fluorescence resonance energy transfer (FRET)-based sensors for 3′⁻5′cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) allow real-time imaging of cAMP levels and kinase activity in intact cells with high spatiotemporal resolution. The development of FRET-based sensors has made it possible to directly demonstrate that cAMP and PKA signals are compartmentalized. These sensors are currently widely used to dissect the organization and physiological function of local cAMP/PKA signaling events in a variety of cell systems. Fusion to targeting domains has been used to direct the sensors to a specific subcellular nanodomain and to monitor cAMP and PKA activity at specific subcellular sites. Here, we investigate the effects of using the A-kinase anchoring protein 79 (AKAP79) as a targeting domain for cAMP and PKA FRET-based reporters. As AKAP79 interacts with PKA itself, when used as a targeting domain, it can potentially impact on the amplitude and kinetics of the signals recorded locally. By using as the targeting domain wild type AKAP79 or a mutant that cannot interact with PKA, we establish that AKAP79 does not affect the amplitude and kinetics of cAMP changes or the level of PKA activity detected by the sensor.

PDE2A2 regulates mitochondria morphology and apoptotic cell death via local modulation of cAMP/PKA signalling

cAMP/PKA signalling is compartmentalised with tight spatial and temporal control of signal propagation underpinning specificity of response. The cAMP-degrading enzymes, phosphodiesterases (PDEs), localise to specific subcellular domains within which they control local cAMP levels and are key regulators of signal compartmentalisation. Several components of the cAMP/PKA cascade are located to different mitochondrial sub-compartments, suggesting the presence of multiple cAMP/PKA signalling domains within the organelle. The function and regulation of these domains remain largely unknown. Here, we describe a novel cAMP/PKA signalling domain localised at mitochondrial membranes and regulated by PDE2A2. Using pharmacological and genetic approaches combined with real-time FRET imaging and high resolution microscopy, we demonstrate that in rat cardiac myocytes and other cell types mitochondrial PDE2A2 regulates local cAMP levels and PKA-dependent phosphorylation of Drp1. We further demonstrate that inhibition of PDE2A, by enhancing the hormone-dependent cAMP response locally, affects mitochondria dynamics and protects from apoptotic cell death.

EPAC1 activation by cAMP stabilizes CFTR at the membrane by promoting its interaction with NHERF1

Cyclic AMP (cAMP) activates protein kinase A (PKA) but also the guanine nucleotide exchange factor 'exchange protein directly activated by cAMP' (EPAC1; also known as RAPGEF3). Although phosphorylation by PKA is known to regulate CFTR channel gating - the protein defective in cystic fibrosis - the contribution of EPAC1 to CFTR regulation remains largely undefined. Here, we demonstrate that in human airway epithelial cells, cAMP signaling through EPAC1 promotes CFTR stabilization at the plasma membrane by attenuating its endocytosis, independently of PKA activation. EPAC1 and CFTR colocalize and interact through protein adaptor NHERF1 (also known as SLC9A3R1). This interaction is promoted by EPAC1 activation, triggering its translocation to the plasma membrane and binding to NHERF1. Our findings identify a new CFTR-interacting protein and demonstrate that cAMP activates CFTR through two different but complementary pathways - the well-known PKA-dependent channel gating pathway and a new mechanism regulating endocytosis that involves EPAC1. The latter might constitute a novel therapeutic target for treatment of cystic fibrosis.

Effect of chronic intravenous injection of steroid hormones on body weight and composition of female rats

The effects of the constant infusion with mini-osmotic pumps of several steroid hormones on body weight, energy balance and protein/lipid/water composition in young female rats has been studied for a period of 15 days. Despite unchanged food consumption, progesterone strongly induced fat deposition, with higher protein accrual efficiency coupled with lowered energy losses through thermogenesis. Estrogens lowered body weight but maintained higher protein levels and protein accrual rates; beta-estradiol induced the loss of lipid and diminished food intake. Heat production was unchanged or lower in all estrogen-treated animals; beta-estradiol had a more marked effect on body weight (through food intake, heat production and lipid mobilization/storage combined) than estrone. Testosterone and 5-androstenediol increased the proportion of protein, but none of them had a significant effect on lipid deposition or heat production. Nortestosterone, increased energy expenditure, fuelled in part by a higher food ingestion, a trait shared by 4-androstenedione, but not by the other androgens. The effect of androgens on body weight may thus be a combination of their actions on a) food intake, b) efficiency of protein deposition and c) activation of heat production or of lipid (energy) storage. Practically all increased the efficiency of protein deposition. Nortestosterone increased heat production. Androstenedione increased lipid storage. Dehydroepiandrosterone did not decrease body weight or metabolic rate. Cortisol depressed heat production and food intake, with a net loss of weight. Cortisol and cortisone did not increase protein deposition, but corticosterone did; deoxycorticosterone showed a high efficiency of protein deposition and increased the size of fat stores, also increasing the metabolic rate by a mean 26% versus controls, compared with a reduction of about the same magnitude induced by cortisol. The data presented suggest that cortisol-cortisone and corticosterone may represent two distinct groups of glucocorticoids.