The flexibility and dynamics of the tubules in the endoplasmic reticulum

Abstract

The endoplasmic reticulum (ER) is a single organelle in eukaryotic cells that extends throughout the cell and is involved in a large number of cellular functions. Using a combination of fixed and live cells (human MRC5 lung cells) in diffraction limited and super-resolved fluorescence microscopy (STORM) experiments, we determined that the average persistence length of the ER tubules was 3.03 ± 0.24 μm. Removing the branched network junctions from the analysis caused a slight increase in the average persistence length to 4.71 ± 0.14 μm, and provides the tubule’s persistence length with a moderate length scale dependence. The average radius of the tubules was 44.1 ± 3.2 nm. The bending rigidity of the ER tubule membranes was found to be 10.9 ± 1.2 kT (17.0 ± 1.3 kT without branch points). We investigated the dynamic behaviour of ER tubules in live cells, and found that the ER tubules behaved like semi-flexible fibres under tension. The majority of the ER tubules experienced equilibrium transverse fluctuations under tension, whereas a minority number of them had active super-diffusive motions driven by motor proteins. Cells thus actively modulate the dynamics of the ER in a well-defined manner, which is expected in turn to impact on its many functions.

Network Organisation and the Dynamics of Tubules in the Endoplasmic Reticulum

“…The videos of ER dynamics in MRC5 cells and the methods used to obtain them was described previously (2). 48 hours prior to imaging, Vero cells were cultured on glass-bottomed 35 mm dishes (μ-dish, No 1.5coverslip, uncoated, Ibidi, Germany).…”

“…Fluorescence images of live cells were used to calculate the persistence length of ER tubules in Vero cells, using a similar method to Georgiades et al (2). The persistence length gives a measure of the flexibility of fibre-like objects.…”

“…The endoplasmic reticulum (ER) is a complex membranous network of tubules and sheet-like regions that is continuous with the nuclear envelope and stretches out to the cell periphery. Narrow tubules (~100 nm in diameter (1,2)) intersect at branch points, which typically connect three tubules. The small diameter of the tubules and their relatively rapid dynamics mean that detailed quantitative analysis has only become possible with recent developments in microscopy, such as super-resolution fluorescence microscopy (3).…”

“…The tubules and junctions of the ER network are highly dynamic and oscillate over time (2,3). The functionality of these oscillations is not yet known, although it has been suggested that mixing of reactants is facilitated by the motion in a shaken reaction vessel model (2). This motion is hypothesized to increase the rate of reactions and therefore the rate of protein biosynthesis.…”

Geometric coupling of helicoidal ramps and curvature-inducing proteins in organelle membranes

“…For instance budding/necking processes occur in the plasma membrane through a variety of endocytic and exocytic mechanisms (e.g., clathrin mediated [6][7][8], ESCRT [9][10][11], antimicrobial peptides [12,13]), but also in the endoplasmic reticulum (ER) and Golgi in anterograde and retrograde trafficking [14,15]. Similarly membrane tubes are present in the ER membrane network [1,[16][17][18], mitochondria [19][20][21], and in the form of cellular nanotubes [22][23][24]. Common to these seemingly disparate membrane structures is the catenoid-like geometry appearing at the neck of buds and the base of tubes [25].…”