Measurement of lateral transport on cell surfaces

Abstract

This paper describes the principles and recent applications of two new methods for measuring rates of macroscopic lateral transport of fluorescent-labeled particles on the surface of individual cells. Both methods are based on microfluorimetric measurements of fluorescence intensity from a small open region (greater than or equal to 1 micronm radius) on the cell surface. Transport rates are measured from the rates at which the measured fluorescence intensity changes due to entrance or departure of fluorophores from the region. One method, "Fluorescence Photobleach Recovery" (FPR), uses a brief intense pulse of light to create an initial concentration gradient over the spot by irreversible photochemical destruction of fluorophores. The rate of fluorescence recovery due to transport of unbleached fluorophores into the observation region is the primary experimental datum. The other method, "Fluorescence Correlation Spectroscopy" (FCS), is based on a statistical analysis of spontaneous fluctuations of numbers of fluorophores in the observation region and does not require a perturbation to generate an initial concentration gradient. FCS is mainly useful to measure relatively fast processes (D greater than or equal to 10(-9) cm 2/sec) in stable systems; FPR can be used to measure both slow and fast transport in less stable systems. Using both FCS and FPR, the diffusion coefficient of a fluorescent lipid probe in rat myoblast plasma membranes was measured to be D = (9 +/- 4) X 10(-9) cm2/sec over a range of at least 4 micronm. FPR was used to measure the lateral mobility of the fluorescent labeled lectin concanavalin A complexed to myoblast plasma membrane "receptors." A fraction of the complexes were immobile on the time scale of the experiment (D less than 8 x 10(-12) cm2/sec). The remainder of the complexes had effective diffusion coefficients far smaller than expected from the measurements on the lipid probe (8 x 10(-12) less than or equal to D less than or equal to 3.3 x 10(-11) cm2/sec). Moreover the mobility depended on the valence, dose, and time of occupancy of the lectin on the membrane, suggesting that an aggregation of complexes was occurring during the experiment. Cytochalasin B decreased the mobility of complexes, suggesting an influence of microfilaments on the transport process. Neither azide nor colchine affected measured transport rates. These results indicate the operation of constraints on the mobility of the lectin-receptor complexes beyond that exerted by the viscous resistance of the lipid bilayer membrane matrix. The two types of interactions revealed by our current experiments - interactions of the complexes with microfilaments and with each other (aggregation) - seem insufficient to account entirely for the low observed mobility.