GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation

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Abstract

Strategies aimed at mimicking or enhancing the action of the incretin hormone glucagon-like peptide 1 (GLP-1) therapeutically improve glucose-stimulated insulin secretion (GSIS); however, it is not clear whether GLP-1 directly drives insulin secretion in pancreatic islets. Here, we examined the mechanisms by which GLP-1 stimulates insulin secretion in mouse and human islets. We found that GLP-1 enhances GSIS at a half-maximal effective concentration of 0.4 pM. Moreover, we determined that GLP-1 activates PLC, which increases submembrane diacylglycerol and thereby activates PKC, resulting in membrane depolarization and increased action potential firing and subsequent stimulation of insulin secretion. The depolarizing effect of GLP-1 on electrical activity was mimicked by the PKC activator PMA, occurred without activation of PKA, and persisted in the presence of PKA inhibitors, the K_ATP channel blocker tolbutamide, and the L-type Ca²⁺ channel blocker isradipine; however, depolarization was abolished by lowering extracellular Na⁺. The PKC-dependent effect of GLP-1 on membrane potential and electrical activity was mediated by activation of Na⁺-permeable TRPM4 and TRPM5 channels by mobilization of intracellular Ca²⁺ from thapsigargin-sensitive Ca²⁺ stores. Concordantly, GLP-1 effects were negligible in Trpm4 or Trpm5 KO islets. These data provide important insight into the therapeutic action of GLP-1 and suggest that circulating levels of this hormone directly stimulate insulin secretion by β cells.

Authors

Makoto Shigeto, Reshma Ramracheya, Andrei I. Tarasov, Chae Young Cha, Margarita V. Chibalina, Benoit Hastoy, Koenraad Philippaert, Thomas Reinbothe, Nils Rorsman, Albert Salehi, William R. Sones, Elisa Vergari, Cathryn Weston, Julia Gorelik, Masashi Katsura, Viacheslav O. Nikolaev, Rudi Vennekens, Manuela Zaccolo, Antony Galione, Paul R.V. Johnson, Kohei Kaku, Graham Ladds, Patrik Rorsman

PRMT1-mediated methylation of the EGF receptor regulates signaling and cetuximab response

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Abstract

Posttranslational modifications to the intracellular domain of the EGFR are known to regulate EGFR functions; however, modifications to the extracellular domain and their effects remain relatively unexplored. Here, we determined that methylation at R198 and R200 of the EGFR extracellular domain by protein arginine methyltransferase 1 (PRMT1) enhances binding to EGF and subsequent receptor dimerization and signaling activation. In a mouse orthotopic colorectal cancer xenograft model, expression of a methylation-defective EGFR reduced tumor growth. Moreover, increased EGFR methylation sustained signaling activation and cell proliferation in the presence of the therapeutic EGFR monoclonal antibody cetuximab. In colorectal cancer patients, EGFR methylation level also correlated with a higher recurrence rate after cetuximab treatment and reduced overall survival. Together, these data indicate that R198/R200 methylation of the EGFR plays an important role in regulating EGFR functionality and resistance to cetuximab treatment.

Authors

Hsin-Wei Liao, Jung-Mao Hsu, Weiya Xia, Hung-Ling Wang, Ying-Nai Wang, Wei-Chao Chang, Stefan T. Arold, Chao-Kai Chou, Pei-Hsiang Tsou, Hirohito Yamaguchi, Yueh-Fu Fang, Hong-Jen Lee, Heng-Huan Lee, Shyh-Kuan Tai, Mhu-Hwa Yang, Maria P. Morelli, Malabika Sen, John E. Ladbury, Chung-Hsuan Chen, Jennifer R. Grandis, Scott Kopetz, Mien-Chie Hung

Ciliopathy proteins regulate paracrine signaling by modulating proteasomal degradation of mediators

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Abstract

Cilia are critical mediators of paracrine signaling; however, it is unknown whether proteins that contribute to ciliopathies converge on multiple paracrine pathways through a common mechanism. Here, we show that loss of cilopathy-associated proteins Bardet-Biedl syndrome 4 (BBS4) or oral-facial-digital syndrome 1 (OFD1) results in the accumulation of signaling mediators normally targeted for proteasomal degradation. In WT cells, several BBS proteins and OFD1 interacted with proteasomal subunits, and loss of either BBS4 or OFD1 led to depletion of multiple subunits from the centrosomal proteasome. Furthermore, overexpression of proteasomal regulatory components or treatment with proteasomal activators sulforaphane (SFN) and mevalonolactone (MVA) ameliorated signaling defects in cells lacking BBS1, BBS4, and OFD1, in morphant zebrafish embryos, and in induced neurons from Ofd1-deficient mice. Finally, we tested the hypothesis that other proteasome-dependent pathways not known to be associated with ciliopathies are defective in the absence of ciliopathy proteins. We found that loss of BBS1, BBS4, or OFD1 led to decreased NF-κB activity and concomitant IκBβ accumulation and that these defects were ameliorated with SFN treatment. Taken together, our data indicate that basal body proteasomal regulation governs paracrine signaling pathways and suggest that augmenting proteasomal function might benefit ciliopathy patients.

Authors

Yangfan P. Liu, I-Chun Tsai, Manuela Morleo, Edwin C. Oh, Carmen C. Leitch, Filomena Massa, Byung-Hoon Lee, David S. Parker, Daniel Finley, Norann A. Zaghloul, Brunella Franco, Nicholas Katsanis

Disrupting hedgehog and WNT signaling interactions promotes cleft lip pathogenesis

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Abstract

Cleft lip, which results from impaired facial process growth and fusion, is one of the most common craniofacial birth defects. Many genes are known to be involved in the etiology of this disorder; however, our understanding of cleft lip pathogenesis remains incomplete. In the present study, we uncovered a role for sonic hedgehog (SHH) signaling during lip fusion. Mice carrying compound mutations in hedgehog acyltransferase (Hhat) and patched1 (Ptch1) exhibited perturbations in the SHH gradient during frontonasal development, which led to hypoplastic nasal process outgrowth, epithelial seam persistence, and cleft lip. Further investigation revealed that enhanced SHH signaling restricts canonical WNT signaling in the lambdoidal region by promoting expression of genes encoding WNT inhibitors. Moreover, reduction of canonical WNT signaling perturbed p63/interferon regulatory factor 6 (p63/IRF6) signaling, resulting in increased proliferation and decreased cell death, which was followed by persistence of the epithelial seam and cleft lip. Consistent with our results, mutations in genes that disrupt SHH and WNT signaling have been identified in both mice and humans with cleft lip. Collectively, our data illustrate that altered SHH signaling contributes to the etiology and pathogenesis of cleft lip through antagonistic interactions with other gene regulatory networks, including the canonical WNT and p63/IRF6 signaling pathways.

Authors

Hiroshi Kurosaka, Angelo Iulianella, Trevor Williams, Paul A. Trainor

Assembly of the cochlear gap junction macromolecular complex requires connexin 26

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Abstract

Hereditary deafness affects approximately 1 in 2,000 children. Mutations in the gene encoding the cochlear gap junction protein connexin 26 (CX26) cause prelingual, nonsyndromic deafness and are responsible for as many as 50% of hereditary deafness cases in certain populations. Connexin-associated deafness is thought to be the result of defective development of auditory sensory epithelium due to connexion dysfunction. Surprisingly, CX26 deficiency is not compensated for by the closely related connexin CX30, which is abundantly expressed in the same cochlear cells. Here, using two mouse models of CX26-associated deafness, we demonstrate that disruption of the CX26-dependent gap junction plaque (GJP) is the earliest observable change during embryonic development of mice with connexin-associated deafness. Loss of CX26 resulted in a drastic reduction in the GJP area and protein level and was associated with excessive endocytosis with increased expression of caveolin 1 and caveolin 2. Furthermore, expression of deafness-associated CX26 and CX30 in cell culture resulted in visible disruption of GJPs and loss of function. Our results demonstrate that deafness-associated mutations in CX26 induce the macromolecular degradation of large gap junction complexes accompanied by an increase in caveolar structures.

Authors

Kazusaku Kamiya, Sabrina W. Yum, Nagomi Kurebayashi, Miho Muraki, Kana Ogawa, Keiko Karasawa, Asuka Miwa, Xueshui Guo, Satoru Gotoh, Yoshinobu Sugitani, Hitomi Yamanaka, Shioko Ito-Kawashima, Takashi Iizuka, Takashi Sakurai, Tetsuo Noda, Osamu Minowa, Katsuhisa Ikeda

Disruption of CEP290 microtubule/membrane-binding domains causes retinal degeneration

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Abstract

Mutations in the gene centrosomal protein 290 kDa (CEP290) cause an array of debilitating and phenotypically distinct human diseases, ranging from the devastating blinding disease Leber congenital amaurosis (LCA) to Senior-Løken syndrome, Joubert syndrome, and the lethal Meckel-Gruber syndrome. Despite its critical role in biology and disease, very little is known about CEP290’s function. Here, we have identified 4 functional domains of the protein. We found that CEP290 directly binds to cellular membranes through an N-terminal domain that includes a highly conserved amphipathic helix motif and to microtubules through a domain located within its myosin-tail homology domain. Furthermore, CEP290 activity was regulated by 2 autoinhibitory domains within its N and C termini, both of which were found to play critical roles in regulating ciliogenesis. Disruption of the microtubule-binding domain in a mouse model of LCA was sufficient to induce significant deficits in cilium formation, which led to retinal degeneration. These data implicate CEP290 as an integral structural and regulatory component of the cilium and provide insight into the pathological mechanisms of LCA and related ciliopathies. Further, these data illustrate that disruption of particular CEP290 functional domains may lead to particular disease phenotypes and suggest innovative strategies for therapeutic intervention.

Authors

Theodore G. Drivas, Erika L.F. Holzbaur, Jean Bennett

p16^INK4a protects against dysfunctional telomere–induced ATR-dependent DNA damage responses

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Abstract

Dysfunctional telomeres limit cellular proliferative capacity by activating the p53-p21– and p16^INK4a-Rb–dependent DNA damage responses (DDRs). The p16^INK4a tumor suppressor accumulates in aging tissues, is a biomarker for cellular senescence, and limits stem cell function in vivo. While the activation of a p53-dependent DDR by dysfunctional telomeres has been well documented in human cells and mouse models, the role for p16^INK4a in response to telomere dysfunction remains unclear. Here, we generated protection of telomeres 1b p16^–/– mice (Pot1b^Δ/Δ;p16^–/–) to address the function of p16^INK4a in the setting of telomere dysfunction in vivo. We found that deletion of p16^INK4a accelerated organ impairment and observed functional defects in highly proliferative organs, including the hematopoietic system, small intestine, and testes. Pot1b^Δ/Δ;p16^–/– hematopoietic cells exhibited increased telomere loss, increased chromosomal fusions, and telomere replication defects. p16^INK4a deletion enhanced the activation of the ATR-dependent DDR in Pot1b^Δ/Δ hematopoietic cells, leading to p53 stabilization, increased p21-dependent cell cycle arrest, and elevated p53-dependent apoptosis. In contrast to p16^INK4a, deletion of p21 did not activate ATR, rescued proliferative defects in Pot1b^Δ/Δ hematopoietic cells, and significantly increased organismal lifespan. Our results provide experimental evidence that p16^INK4a exerts protective functions in proliferative cells bearing dysfunctional telomeres.

Authors

Yang Wang, Norman Sharpless, Sandy Chang

A noninhibitory mutant of the caveolin-1 scaffolding domain enhances eNOS-derived NO synthesis and vasodilation in mice

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Abstract

Aberrant regulation of eNOS and associated NO release are directly linked with various vascular diseases. Caveolin-1 (Cav-1), the main coat protein of caveolae, is highly expressed in endothelial cells. Its scaffolding domain serves as an endogenous negative regulator of eNOS function. Structure-function analysis of Cav-1 has shown that phenylalanine 92 (F92) is critical for the inhibitory actions of Cav-1 toward eNOS. Herein, we show that F92A–Cav-1 and a mutant cell–permeable scaffolding domain peptide called Cavnoxin can increase basal NO release in eNOS-expressing cells. Cavnoxin reduced vascular tone ex vivo and lowered blood pressure in normal mice. In contrast, similar experiments performed with eNOS- or Cav-1–deficient mice showed that the vasodilatory effect of Cavnoxin is abolished in the absence of these gene products, which indicates a high level of eNOS/Cav-1 specificity. Mechanistically, biochemical assays indicated that noninhibitory F92A–Cav-1 and Cavnoxin specifically disrupted the inhibitory actions of endogenous Cav-1 toward eNOS and thereby enhanced basal NO release. Collectively, these data raise the possibility of studying the inhibitory influence of Cav-1 on eNOS without interfering with the other actions of endogenous Cav-1. They also suggest a therapeutic application for regulating the eNOS/Cav-1 interaction in diseases characterized by decreased NO release.

Authors

Pascal Bernatchez, Arpeeta Sharma, Philip M. Bauer, Ethan Marin, William C. Sessa

Engagement of S1P₁-degradative mechanisms leads to vascular leak in mice

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Abstract

GPCR inhibitors are highly prevalent in modern therapeutics. However, interference with complex GPCR regulatory mechanisms leads to both therapeutic efficacy and adverse effects. Recently, the sphingosine-1-phosphate (S1P) receptor inhibitor FTY720 (also known as Fingolimod), which induces lymphopenia and prevents neuroinflammation, was adopted as a disease-modifying therapeutic in multiple sclerosis. Although highly efficacious, dose-dependent increases in adverse events have tempered its utility. We show here that FTY720P induces phosphorylation of the C-terminal domain of S1P receptor 1 (S1P1) at multiple sites, resulting in GPCR internalization, polyubiquitinylation, and degradation. We also identified the ubiquitin E3 ligase WWP2 in the GPCR complex and demonstrated its requirement in FTY720-induced receptor degradation. GPCR degradation was not essential for the induction of lymphopenia, but was critical for pulmonary vascular leak in vivo. Prevention of receptor phosphorylation, internalization, and degradation inhibited vascular leak, which suggests that discrete mechanisms of S1P receptor regulation are responsible for the efficacy and adverse events associated with this class of therapeutics.

Authors

Myat Lin Oo, Sung-Hee Chang, Shobha Thangada, Ming-Tao Wu, Karim Rezaul, Victoria Blaho, Sun-Il Hwang, David K. Han, Timothy Hla

Intoxication of zebrafish and mammalian cells by cholera toxin depends on the flotillin/reggie proteins but not Derlin-1 or -2

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Abstract

Cholera toxin (CT) causes the massive secretory diarrhea associated with epidemic cholera. To induce disease, CT enters the cytosol of host cells by co-opting a lipid-based sorting pathway from the plasma membrane, through the trans-Golgi network (TGN), and into the endoplasmic reticulum (ER). In the ER, a portion of the toxin is unfolded and retro-translocated to the cytosol. Here, we established zebrafish as a genetic model of intoxication and examined the Derlin and flotillin proteins, which are thought to be usurped by CT for retro-translocation and lipid sorting, respectively. Using antisense morpholino oligomers and siRNA, we found that depletion of Derlin-1, a component of the Hrd-1 retro-translocation complex, was dispensable for CT-induced toxicity. In contrast, the lipid raft–associated proteins flotillin-1 and -2 were required. We found that in mammalian cells, CT intoxication was dependent on the flotillins for trafficking between plasma membrane/endosomes and two pathways into the ER, only one of which appears to intersect the TGN. These results revise current models for CT intoxication and implicate protein scaffolding of lipid rafts in the endosomal sorting of the toxin-GM1 complex.

Authors

David E. Saslowsky, Jin Ah Cho, Himani Chinnapen, Ramiro H. Massol, Daniel J.-F. Chinnapen, Jessica S. Wagner, Heidi E. De Luca, Wendy Kam, Barry H. Paw, Wayne I. Lencer