Simon A. Hinke

AKAP150 Contributes to Enhanced Vascular Tone by Facilitating Large-Conductance Ca2+-Activated K+ Channel Remodeling in Hyperglycemia and Diabetes Mellitus

Rationale: Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes mellitus may arise from impaired large-conductance Ca2+-activated K+ (BKCa) channel function. The scaffolding protein A-kinase anchoring protein 150 (AKAP150) is a key regulator of calcineurin (CaN), a phosphatase known to modulate the expression of the regulatory BKCa &bgr;1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes mellitus is unknown. Objective: To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa &bgr;1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes mellitus. Methods and Results: We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/nuclear factor of activated T-cells c3 (NFATc3) activation, and resistance artery constriction in hyperglycemic animals on high-fat diet. Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. D-glucose–dependent suppression of BKCa channel &bgr;1 subunits required Ca2+ influx via voltage-gated L-type Ca2+ channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, high-fat diet mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa &bgr;1 subunits and attenuated high-fat diet–induced elevation in arterial blood pressure. Conclusions: Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes mellitus.

MyRIP Anchors Protein Kinase A to the Exocyst Complex

The movement of signal transduction enzymes in and out of multi-protein complexes coordinates the spatial and temporal resolution of cellular events. Anchoring and scaffolding proteins are key to this process because they sequester protein kinases and phosphatases with a subset of their preferred substrates. The protein kinase A-anchoring family of proteins (AKAPs), which target the cAMP-dependent protein kinase (PKA) and other enzymes to defined subcellular microenvironments, represent a well studied group of these signal-organizing molecules. In this report we demonstrate that the Rab27a GTPase effector protein MyRIP is a member of the AKAP family. The zebrafish homolog of MyRIP (Ze-AKAP2) was initially detected in a two-hybrid screen for AKAPs. A combination of biochemical, cell-based, and immunofluorescence approaches demonstrate that the mouse MyRIP ortholog targets the type II PKA holoenzyme via an atypical mechanism to a specific perinuclear region of insulin-secreting cells. Similar approaches show that MyRIP interacts with the Sec6 and Sec8 components of the exocyst complex, an evolutionarily conserved protein unit that controls protein trafficking and exocytosis. These data indicate that MyRIP functions as a scaffolding protein that links PKA to components of the exocytosis machinery.

Anchored phosphatases modulate glucose homeostasis

Endocrine release of insulin principally controls glucose homeostasis. Nutrient‐induced exocytosis of insulin granules from pancreatic β‐cells involves ion channels and mobilization of Ca2+ and cyclic AMP (cAMP) signalling pathways. Whole‐animal physiology, islet studies and live‐β‐cell imaging approaches reveal that ablation of the kinase/phosphatase anchoring protein AKAP150 impairs insulin secretion in mice. Loss of AKAP150 impacts L‐type Ca2+ currents, and attenuates cytoplasmic accumulation of Ca2+ and cAMP in β‐cells. Yet surprisingly AKAP150 null animals display improved glucose handling and heightened insulin sensitivity in skeletal muscle. More refined analyses of AKAP150 knock‐in mice unable to anchor protein kinase A or protein phosphatase 2B uncover an unexpected observation that tethering of phosphatases to a seven‐residue sequence of the anchoring protein is the predominant molecular event underlying these metabolic phenotypes. Thus anchored signalling events that facilitate insulin secretion and glucose homeostasis may be set by AKAP150 associated phosphatase activity.

AKAP150 participates in calcineurin/NFAT activation during the down-regulation of voltage-gated K+ currents in ventricular myocytes following myocardial infarction

The Ca(2+)-responsive phosphatase calcineurin/protein phosphatase 2B dephosphorylates the transcription factor NFATc3. In the myocardium activation of NFATc3 down-regulates the expression of voltage-gated K(+) (Kv) channels after myocardial infarction (MI). This prolongs action potential duration and increases the probability of arrhythmias. Although recent studies infer that calcineurin is activated by local and transient Ca(2+) signals the molecular mechanism that underlies the process is unclear in ventricular myocytes. Here we test the hypothesis that sequestering of calcineurin to the sarcolemma of ventricular myocytes by the anchoring protein AKAP150 is required for acute activation of NFATc3 and the concomitant down-regulation of Kv channels following MI. Biochemical and cell based measurements resolve that approximately 0.2% of the total calcineurin activity in cardiomyocytes is associated with AKAP150. Electrophysiological analyses establish that formation of this AKAP150-calcineurin signaling dyad is essential for the activation of the phosphatase and the subsequent down-regulation of Kv channel currents following MI. Thus AKAP150-mediated targeting of calcineurin to sarcolemmal micro-domains in ventricular myocytes contributes to the local and acute gene remodeling events that lead to the down-regulation of Kv currents.

Epac2: A Molecular Target for Sulfonylurea-Induced Insulin Release

Epac2 is an intracellular receptor for a commonly used class of antidiabetic medications. Sulfonylurea drugs are used in type 2 diabetes mellitus therapy to induce release of endogenous insulin from pancreatic β cells. They act on sulfonylurea receptors, which are the regulatory subunits of adenosine triphosphate (ATP)–sensitive potassium ion (KATP) channels and cause channel closure to trigger exocytosis. Epac2 was identified as an intracellular target for sulfonylurea drugs, providing a potential nonelectrogenic signaling component to the mechanism of action for these agents. Commonly used sulfonylureas such as tolbutamide and glibenclamide induced Epac2 activation with distinct kinetic profiles. Epac2−/− mice failed to respond to sulfonylureas, suggesting that both sulfonylurea receptors and Epac2 are necessary for the action of these drugs. These data require that the cellular and physiological effects of drugs that alter the open state of the KATP channel be reassessed.

Inverse vaccination with islet autoantigens to halt progression of autoimmune diabetes

The incidence of autoimmune diabetes mellitus is growing worldwide; currently the only treatment is injection of insulin formulations to maintain glycemic control. Islet transplantation has been examined as a potential cure for diabetes. However, it has several limitations, including side effects of immunosuppressive agents required to prevent graft rejection. Other approaches have been to examine immune modulation to halt or prevent the autoimmune attack, with the hope that by preventing tissue damage, it will eventually regenerate and permit patients to be free from insulin injections. The focus of the current review is the three major antigen‐based vaccines, insulin, glutamic acid decarboxylase 65 (GAD65; Diamyd), and heat shock protein 60 (Hsp60; DiaPep277), with respect to their mechanism of action, preclinical results, and published results from clinical trials. Despite promising results with these three antigens in mice and humans, efficacy has been limited in clinical trials. Drug Dev Res 72:788–804, 2011.

AKAP150 Contributes to Enhanced Vascular Tone by Facilitating Large-Conductance Ca2+-Activated K+ Channel Remodeling in Hyperglycemia and Diabetes MellitusNovelty and Significance

Rationale: Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes mellitus may arise from impaired large-conductance Ca2+-activated K+ (BKCa) channel function. The scaffolding protein A-kinase anchoring protein 150 (AKAP150) is a key regulator of calcineurin (CaN), a phosphatase known to modulate the expression of the regulatory BKCa &bgr;1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes mellitus is unknown. Objective: To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa &bgr;1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes mellitus. Methods and Results: We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/nuclear factor of activated T-cells c3 (NFATc3) activation, and resistance artery constriction in hyperglycemic animals on high-fat diet. Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. D-glucose–dependent suppression of BKCa channel &bgr;1 subunits required Ca2+ influx via voltage-gated L-type Ca2+ channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, high-fat diet mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa &bgr;1 subunits and attenuated high-fat diet–induced elevation in arterial blood pressure. Conclusions: Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes mellitus.

AKAP150 Contributes to Enhanced Vascular Tone by Facilitating BKCa Channel Remodeling in Hyperglycemia and Diabetes

Rationale: Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes mellitus may arise from impaired large-conductance Ca2+-activated K+ (BKCa) channel function. The scaffolding protein A-kinase anchoring protein 150 (AKAP150) is a key regulator of calcineurin (CaN), a phosphatase known to modulate the expression of the regulatory BKCa &bgr;1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes mellitus is unknown. Objective: To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa &bgr;1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes mellitus. Methods and Results: We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/nuclear factor of activated T-cells c3 (NFATc3) activation, and resistance artery constriction in hyperglycemic animals on high-fat diet. Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. D-glucose–dependent suppression of BKCa channel &bgr;1 subunits required Ca2+ influx via voltage-gated L-type Ca2+ channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, high-fat diet mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa &bgr;1 subunits and attenuated high-fat diet–induced elevation in arterial blood pressure. Conclusions: Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes mellitus.

Erratum: Anchored phosphatases modulate glucose homeostasis (EMBO Journal (2012) 31 (3991-4004) DOI: 10.1038/emboj.2012.244)

The EMBO Journal (2012) 31, 4481. doi:10.1038/emboj.2012.297 Correction to: The EMBO Journal (2012) 31, 3991–4004. doi:10.1038/emboj.2012.244 Since the publication of this paper, the authors have realised that they should have included a further acknowledgement. They would like to acknowledge their grant number DK54441 from the National Institutes of Health. The authors apologise for any inconvenience caused. The EMBO Journal (2012) 31, 4481 | & 2012 European Molecular Biology Organization |All Rights Reserved 0261-4189/12 www.embojournal.org EMBO THE JOURNAL