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ABSTRACTBECN2 is a mammal-specific homolog of BECN1. Both proteins serve as a scaffolding molecule in the class III phosphatidylinositol 3-kinase complex (PtdIns3K) to promote macroautophagy/autophagy and endolysosomal trafficking. Our previous studies have shown that the BECN1 coiled-coil domain forms a metastable homodimer and readily self-dissociate to form the BECN1-ATG14 or BECN1-UVRAG coiled-coil complex as part of the scaffolding “arm” of the PtdIns3K complex. Here we report the crystal structure of the BECN2 coiled-coil domain, which forms a metastable homodimer similar to BECN1 but shows reduced stability and large deviation from the ideal coiled-coil geometry due to extra “imperfect” residues. We also report the crystal structure of BECN2-ATG14 coiled-coil complex, which is structurally similar to the BECN1-UVRAG coiled-coil complex we reported previously but functionally distinct. The potent BECN2-ATG14 interaction is selectively critical for endolysosomal degradation of the GPRASP1-associated DRD2/D2R but shows limited effect on EGFR, a cargo that depends on the BECN1-UVRAG interaction. We designed stapled peptides that selectively interacted with the BECN2 coiled-coil domain to enhance the BECN2-ATG14 or BECN1-UVRAG interaction. One such peptide specifically promoted BECN2-dependent processes including autophagy and endolysosomal degradation of DRD2/D2R but did not affect BECN1-dependent EGFR degradation. Our findings suggest that, despite high sequence identity to BECN1, BECN2 coiled-coil domain has unique structural features and the BECN2-ATG14 interaction selectively promotes autophagic or endolysosomal degradation of BECN2-specific cargos including GPRASP1-associated GPCRs.AbbreviationsAMBRA1 autophagy and beclin 1 regulator 1; ATG14 autophagy related 14; ATG5 autophagy related 5; ATG7 autophagy related 7; BECN1 beclin 1; BECN2 beclin 2; CC coiled-coil; CQ chloroquine CNR1/CB1R cannabinoid receptor 1 DAPI 4ʹ,6-diamidino-2-phenylindole; dCCD delete CCD; DRD2/D2R dopamine receptor D2 GPRASP1/GASP1 G protein-coupled receptor associated sorting protein 1 GPCR G-protein coupled receptor; ITC isothermal titration calorimetry; IP immunoprecipitation; KD knockdown; KO knockout; MAP1LC3/LC3 microtubule associated protein 1 light chain 3; NRBF2 nuclear receptor binding factor 2; OPRD1/DOR opioid receptor delta 1 PIK3C3/VPS34 phosphatidylinositol 3-kinase catalytic subunit type 3; PIK3R4/VPS15 phosphoinositide-3-kinase regulatory subunit 4; PtdIns3K class III phosphatidylinositol 3-kinase; PtdIns3P phosphatidylinositol-3-phosphate; RUBCN rubicon autophagy regulator; SQSTM1/p62 sequestosome 1; UVRAG UV radiation resistance associated; VPS vacuolar protein sorting; WT wild type.

Autophagy, a stress-induced lysosomal degradative pathway, has been assumed to exert similar metabolic effects in different organs. Here, we establish a model where autophagy plays different roles in insulin-producing β cells versus insulin-responsive cells, utilizing knockin (Becn1F121A) mice manifesting constitutively active autophagy. With a high-fat-diet challenge, the autophagy-hyperactive mice unexpectedly show impaired glucose tolerance, but improved insulin sensitivity, compared to mice with normal autophagy. Autophagy hyperactivation enhances insulin signaling, via suppressing ER stress in insulin-responsive cells, but decreases insulin secretion by selectively sequestrating and degrading insulin granule vesicles in β cells, a process we term “vesicophagy.” The reduction in insulin storage, insulin secretion, and glucose tolerance is reversed by transient treatment of autophagy inhibitors. Thus, β cells and insulin-responsive tissues require different autophagy levels for optimal function. To improve insulin sensitivity without hampering secretion, acute or intermittent, rather than chronic, activation of autophagy should be considered in diabetic therapy development.

ABSTRACTTigecycline (TGC) is a last-line drug for multidrug-resistant Enterobacteriaceae. We investigated the mechanism(s) underlying TGC nonsusceptibility (TGC resistant/intermediate) in Escherichia coliclinical isolates. The MIC of TGC was determined for 277 fluoroquinolone-susceptible isolates (ciprofloxacin [CIP] MIC, <0.125 mg/liter) and 194 fluoroquinolone-resistant isolates (CIP MIC, >2 mg/liter). The MIC50and MIC90for TGC in fluoroquinolone-resistant isolates were 2-fold higher than those in fluoroquinolone-susceptible isolates (MIC50, 0.5 mg/liter versus 0.25 mg/liter; MIC90, 1 mg/liter versus 0.5 mg/liter, respectively). Two fluoroquinolone-resistant isolates (O25b:H4-ST131-H30R and O125:H37-ST48) were TGC resistant (MICs of 4 and 16 mg/liter, respectively), and four other isolates of O25b:H4-ST131-H30R and an isolate of O1-ST648 showed an intermediate interpretation (MIC, 2 mg/liter). No TGC-resistant/intermediate strains were found among the fluoroquinolone-susceptible isolates. The TGC-resistant/intermediate isolates expressed higher levels of acrAand acrBand had lower intracellular TGC concentrations than susceptible isolates, and they possessed mutations in acrRand/or marR. The MICs of acrAB-deficient mutants were markedly lower (0.25 mg/liter) than those of the parental strain. After continuous stepwise exposure to CIP in vitro, six of eight TGC-susceptible isolates had reduced TGC susceptibility. Two of them acquired TGC resistance (TGC MIC, 4 mg/liter) and exhibited expression of acrAand acrBand mutations in acrRand/or marR. In conclusion, a population of fluoroquinolone-resistant E. coliisolates, including major extraintestinal pathogenic lineages O25b:H4-ST131-H30R and O1-ST648, showed reduced susceptibility to TGC due to overexpression of the efflux pump AcrAB-TolC, leading to decreased intracellular concentrations of the antibiotics that may be associated with the development of fluoroquinolone resistance.