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Quantitative Measurement of GLUT4 Translocation to the Plasma Membrane by Flow Cytometry

Glucose is the main source of energy for the body, requiring constant regulation of its blood concentration. Insulin release by the pancreas induces glucose uptake by insulin-sensitive tissues, most notably the brain, skeletal muscle, and adipocytes. Patients suffering from type-2 diabetes and/or obesity often develop insulin resistance and are unable to control their glucose homeostasis. New insights into the mechanisms of insulin resistance may provide new treatment strategies for type-2 diabetes. The GLUT family of glucose transporters consists of thirteen members distributed on different tissues throughout the body1. Glucose transporter type 4 (GLUT4) is the major transporter that mediates glucose uptake by insulin sensitive tissues, such as the skeletal muscle. Upon binding of insulin to its receptor, vesicles containing GLUT4 translocate from the cytoplasm to the plasma membrane, inducing glucose uptake. Reduced GLUT4 translocation is one of the causes of insulin resistance in type-2 diabetes2,3. The translocation of GLUT4 from the cytoplasm to the plasma membrane can be visualized by immunocytochemistry, using fluorophore-conjugated GLUT4-specific antibodies. Here, we describe a technique to quantify total amounts of GLUT4 translocation to the plasma membrane of cells during a chosen duration, using flow cytometry. This protocol is rapid (less than 4 hours, including incubation with insulin) and allows the analysis of as few as 3,000 cells or as many as 1 million cells per condition in a single experiment. It relies on anti-GLUT4 antibodies directed to an external epitope of the transporter that bind to it as soon as it is exposed to the extracellular medium after translocation to the plasma membrane.


1
. Glucose transporter type 4 (GLUT4) is the major transporter that mediates glucose uptake by insulin sensitive tissues, such as the skeletal muscle. Upon binding of insulin to its receptor, vesicles containing GLUT4 translocate from the cytoplasm to the plasma membrane, inducing glucose uptake. Reduced GLUT4 translocation is one of the causes of insulin resistance in type-2 diabetes 2,3 .
The translocation of GLUT4 from the cytoplasm to the plasma membrane can be visualized by immunocytochemistry, using fluorophoreconjugated GLUT4-specific antibodies.
Here, we describe a technique to quantify total amounts of GLUT4 translocation to the plasma membrane of cells during a chosen duration, using flow cytometry. This protocol is rapid (less than 4 hours, including incubation with insulin) and allows the analysis of as few as 3,000 cells or as many as 1 million cells per condition in a single experiment. It relies on anti-GLUT4 antibodies directed to an external epitope of the transporter that bind to it as soon as it is exposed to the extracellular medium after translocation to the plasma membrane.

Discussion
We have demonstrated a rapid technique for quantifying the translocation of GLUT4 from the cytoplasm to the plasma membrane of cells by flow cytometry.
GLUT4 is only transiently expressed at the plasma membrane of cells and is endocytosed. Since the bound antibodies remain attached to the GLUT4, even during endocytosis, the fluorescence signal gives the total amount of GLUT4 that was exposed at the plasma membrane during the chosen duration of incubation in the presence of insulin.
This technique can also be applied to studying the translocation of other proteins from an intracellular compartment to the plasma membrane, provided a good antibody directed to an external epitope of the protein of interest is available. For example, a similar assay is now commonly used to assess the rate of degranulation of cytotoxic T lymphocytes and natural killer lymphocytes by measuring the translocation of CD107a to their plasma membrane 4,5 . The translocation of phosphatidylserine from the inner side of the plasma membrane to the outer side during cell apoptosis or necrosis can also be detected by flow cytometry, using fluorophore-labeled Annexin V 6 .
Besides rapidity of the assay, flow cytometry presents the advantage of allowing the study of protein translocation to the plasma membrane in mixed cell populations. Indeed, one can combine staining for cell subset markers and for the protein of interest, followed by data acquisition on a multi-laser flow cytometer 7 .
The amounts of antibodies given in paragraph 1.2 of the protocol are a starting point; we recommend you titrate your antibodies with your cells of interest to determine the minimum antibody concentration that gives you a maximum signal. A good starting range for the primary anti-human GLUT4 antibody we have used here would be 1-10 μL per tube (i.e. 0.2-2 μg antibody per tube), each tube containing no more than 1 million cells. For other primary antibodies, refer to the supplier's instructions.
Normalization of data is necessary as autofluorescence of cells will vary from donor to donor and from experiment to experiment.

Disclosures
No conflicts of interest declared.