Miscibility Transition Temperature Scales with Growth Temperature in a Zebrafish Cell Line

Abstract

Cells can alter the lipid content of their plasma membranes upon changes in their environment to maintain and adjust membrane function. Recent work suggests that some membrane functions arise because cellular plasma membranes are poised close to a miscibility transition under growth conditions. Here we report experiments utilizing giant plasma membrane vesicles (GPMVs) to explore how membrane transition temperature varies with growth temperature in a zebrafish cell line (ZF4) that can be adapted for growth between 20 and 32°C. We find that GPMV transition temperatures adjust to be 16.7 ± 1.2°C below growth temperature for four growth temperatures investigated and that adjustment occurs over roughly 2 days when temperature is abruptly lowered from 28 to 20°C. We also find that GPMVs have slightly different lipidomes when isolated from cells adapted for growth at 28 and 20°C. Similar to past work in vesicles derived from mammalian cells, fluctuating domains are observed in ZF4-derived GPMVs, consistent with their having critical membrane compositions. Taken together, these experimental results suggest that cells in culture biologically tune their membrane composition in a way that maintains specific proximity to a critical miscibility transition.

Figure 1

GPMV transition temperatures shift with growth temperatures in the ZF4 cell line. (A) Shown here are representative fields of DiI-C₁₂ labeled GPMVs isolated from ZF4 cells adapted for growth at 28°C and imaged at different temperatures. The fluorophore partitions strongly into the liquid-disordered phase, and phase-separated vesicles are identified by monitoring the lateral distribution of this probe. Phase-separated GPMVs are indicated with yellow arrows and the scale bar represents 50 μm. (B) (Left) Multiple images like the ones shown in (A) are acquired at several fixed temperatures and are used to tabulate the percentage of GPMVs that contain coexisting liquid phases as a function of temperature. These curves are fit to a sigmoid function to extract the midpoint of the transition within the sample, defining T_mix. (Right) Curves from cells grown at 28°C but isolated on different days. These show systematic differences in the fraction of phase-separated vesicles as a function of temperature. This manifests itself as T_mix having some day-to-day variation, which in this example spans roughly 7°C. (C) Shown here are curves like those shown in (B) but generated from GPMVs from cells adapted for growth at different temperatures as indicated in the legend. (D) Shown here are average values of GPMV T_mix plotted as a function of growth temperature. The fit is to a line with a slope of 1.1 ± 0.1°C. The same symbols and colors are used to depict growth temperatures in (B)–(D). To see this figure in color, go online.

Figure 2

Images of ZF4-derived GMPVs. (A) Shown here are representative GPMVs displaying different domain morphologies. Domains, when present, are more likely to have rough and fluctuating boundaries when temperature is above or close to the miscibility transition temperature for the specific vesicle (T_mix). At lower temperatures, sometimes domains fully coarsen so that each phase takes up a hemisphere of the vesicle. In other cases, domains form stable and regular circular or striped regions characteristic of modulated domains. (B) Shown here are images of a single vesicle imaged over a range of temperatures. Temperature was not carefully monitored in this example, but was varied from above T_mix (far left) to below T_mix (far right). These images are captured from frames within Movie S1. The scale bar represents 10 μm. To see this figure in color, go online.

Figure 3

GPMV transition temperatures adapt to a temperature jump in roughly the same time it takes for cells to double. (A and B) Given here is a representative time-course experiment for cells adapted for growth and plated at 28°C before being moved to a 20°C incubator on day 0 (0 day). Raw curves showing the fraction of vesicles containing two liquid phases are shown in (A) and are used to determine the average values plotted in (B), with the symbols consistent between the two panels. (C) Given here is the variation in T_mix as a function of days in culture averaged over four distinct measurements, including the one shown in (A) and (B). Points are fit to an exponential decay, T_mix(t) = T_mix20 + ΔT_mix × exp(−t/τ), with T_mix20 = 0.7 ± 1.0°C, ΔT_mix = 10.8 ± 1.7°C, and τ = 1.3 ± 0.5 days. (D) For three of the measurements shown in (C), we also counted cells in dishes prepared identically to the ones used for GPMV isolation. We observed a roughly linear increase in the number of cells with increasing time, with the time required for doubling of 1.7 ± 0.2 days. Errors in parameter values reported for (C) and (D) are 68% confidence interval estimates evaluated from the fitting procedure. To see this figure in color, go online.

Figure 4

Summary of lipidomics results for GPMVs isolated from ZF4 adapted for growth at 20 and 28°C. (A–C) Given here is the molar percent of lipid types grouped according to headgroup type, total lipid unsaturation, individual fatty acid chain unsaturation, and fatty acid chain length. Top panels indicate average values acquired for samples isolated from cells grown at both 20 and 28°C, and error bars indicate the mean ± SE over four measurements. Bottom panels indicate the mol % change observed for samples grown at 20 and 28°C, and error bars indicate the mean ± SE over two difference measurements. (D) Given here are the 10 most abundant lipids observed in GPMVs from cells grown at 20°C (left) and 28°C (right). Lipid species not present on both lists are indicated in red text. (E) Shown here are the 10 lipids found at higher abundance in GPMVs from cells grown at 20 vs. 28°C (left) or 28 vs. 20°C, in order of the magnitude of their increased abundance. Lipids indicated in red text are repeated from (D). Lipid species not present in either list in (D) are indicated as green text. To see this figure in color, go online.