Trans-Membrane Area Asymmetry Controls the Shape of Cellular Organelles

Abstract

Membrane organelles often have complicated shapes and differ in their volume, surface area and membrane curvature. The ratio between the surface area of the cytosolic and luminal leaflets (trans-membrane area asymmetry (TAA)) determines the membrane curvature within different sites of the organelle. Thus, the shape of the organelle could be critically dependent on TAA. Here, using mathematical modeling and stereological measurements of TAA during fast transformation of organelle shapes, we present evidence that suggests that when organelle volume and surface area are constant, TAA can regulate transformation of the shape of the Golgi apparatus, endosomal multivesicular bodies, and microvilli of brush borders of kidney epithelial cells. Extraction of membrane curvature by small spheres, such as COPI-dependent vesicles within the Golgi (extraction of positive curvature), or by intraluminal vesicles within endosomes (extraction of negative curvature) controls the shape of these organelles. For instance, Golgi tubulation is critically dependent on the fusion of COPI vesicles with Golgi cisternae, and vice versa, for the extraction of membrane curvature into 50–60 nm vesicles, to induce transformation of Golgi tubules into cisternae. Also, formation of intraluminal ultra-small vesicles after fusion of endosomes allows equilibration of their TAA, volume and surface area. Finally, when microvilli of the brush border are broken into vesicles and microvilli fragments, TAA of these membranes remains the same as TAA of the microvilli. Thus, TAA has a significant role in transformation of organelle shape when other factors remain constant.

Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry

“…We tested the role of the TAA hypothesis in the shaping of cell organelles using three different experimental models, namely: The fusion of small spheres with discs with toroid edges, or the separation of a small sphere from such discs; the fusion of two spheres; and finally, the separation of the cylinder into spheres, when the diffusion of lipids into the cylinder is limited due to low temperatures [ 4 ]. These experiments have a common feature, namely, the delivery of lipid molecules by their rapid diffusion along one of the leaflets biological membranes is limited or severely limited.…”

“…When the level of undulation becomes minimal, further detachment of COPI vesicles transforms a solid disk into a solid invaginated semi-sphere. In contrast, the fusion of COPI-dependent vesicles with a Golgi cisterna would widen cisternal pores and increase the level of rim undulation [ 4 ].…”

“…Additionally, we demonstrated that (i) inhibition of the SNARE machinery alone reduced TAA of the Golgi cisternae, and induced a narrowing of the cisternal perforations, followed by invagination of cisternal membranes; (ii) inhibition of the ARF/COPI machinery alone increased the TAA of the Golgi cisternae, and induced a widening of the cisternal perforations, followed by the Golgi tubulation (it was known that inhibition of the activity of the ARF/COPI machine led to the Golgi tubulation when GC is functionally isolated from the ER, whereas otherwise Golgi markers go to the ER [ 4 ]); and (iii) inhibition of both machineries did not change Golgi shape significantly. In all of these cases TAA remained constant (iv).…”

“…When the membrane is stretched, a hole is easily created in the membrane [ 2 , 3 , 4 ]. At a certain point of the membrane surface, the degree of membrane bending is described in terms of the membrane curvature, and determined by local asymmetries in the size of the areas of two bilayer leaflets [ 3 , 4 , 5 , 6 ]. Here, for simplicity of consideration, we use only ideal models, and deal with the spontaneous membrane curvature, which depends on the bilayer itself.…”

The Step-Wise C-Truncation and Transport of ESyt3 to Lipid Droplets Reveals a Mother Primordial Cisterna

“…The necessary cell fixation with 4% paraformaldehyde and 0.05% glutaraldehyde precluded the use of the anti-E-Syt3 antibodies. Accordingly, pre-embedding immune EM detection of E-Syt3C2C was performed in young adipocytes on day 3 of differentiation using rabbit anti-EGFP antibody and goat anti-rabbit Fab' fragments coupled to 1.4 nm gold particles as described (Kweon et al, 2004) (Beznoussenko et al, 2015). TEM tomograms were reconstructed by acquiring images automatically at a series of specimen tilt using the iterative reconstruction technique implemented by the program Xplore3DTMXpress-Inspect3DTM and the last version of the IMOD package (Mastronarde, 2005).…”