Push/pull mechanisms of GLUT4 traffic in muscle cells.
OBJECTIVE
Understanding the mechanisms by which insulin regulates glucose transporter 4 (GLUT4) traffic in skeletal muscle has been a major challenge since the discoveries of glucose transporter's translocation and the cloning of GLUT4. Here we summarize our work of the past 5 years on the regulation of GLUT4 traffic in skeletal muscle cells.


METHODS
L6 cells overexpressing GLUT4 harbouring an exofacial myc epitope gave us the opportunity to perform dynamic assessments of GLUT4 exocytosis, endocytosis, as well as a means to follow GLUT4 molecules along their journey through intracellular compartments.


RESULTS
We found that insulin stimulation, which results in the expected gain in surface GLUT4, is mostly attributed to enhanced GLUT4 exocytosis, and does not significantly affect the initial rate of internalization. Two mechanisms by which insulin enhances GLUT4 exocytosis are described: 'Pull' relates to actin remodelling-based segregation of the insulin signalling molecules and the directed recruitment of GLUT4/VAMP2 containing vesicles. 'Push' is the accelerated inter-endosomal transit of endocytosed GLUT4 molecules through the recycling endosome. The interface between the two types of regulatory input by insulin is suggested to be the budding of GLUT4 from the transferrin receptor (TfR)-containing, recycling endosome.


CONCLUSIONS
We propose a model on the identity of the GLUT4 pools responsible for GLUT4 recruitment to the plasma membrane in the basal state, or following insulin or hyperosmolarity stimuli.
Rudich
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Programme in Cell Biology, The Hospital for Sick Children, Toronto, Ontario, Canada.
Klip
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D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology; D019469:Indinavir / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D008954:Models, Biological; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D009124:Muscle Proteins; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D050683:R-SNARE Proteins
D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents; D007328:Insulin; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D050683:R-SNARE Proteins; C495692:SLC2A4 protein, human; C495693:Slc2a4 protein, mouse; D019469:Indinavir; D005947:Glucose