Chapter 6: cubic membranes the missing dimension of cell membrane organization

Abstract

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between "inside" and "outside", survival, and death. Intracellular compartmentalization-governed by biomembranes as well-is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into "unusual", highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Figure 6.1

Cubic membrane architecture (Almsherqi et al., 2008). (A) Two-dimensional transmission electron micrograph of a mitochondrion of 10 days starved amoeba Chaos cells and (B) three-dimensional mathematical model of the same type of cubic membrane organization. Scale bar: 250 nm.

Figure 6.2

Periodic cubic surfaces and cubic membrane. Oblique views of the unit cell of (A) Primitive, (B) Double Diamond, and (C) Gyroid cubic surfaces observed in biological systems. (D) The bilayer constellation of a 3D mathematical model of a cubic membrane. Three parallel Gyroid-based surfaces can be used to describe a biological membrane (bilayer), in which case the centered surface is the “imaginary” hydrophobic mid-bilayer surface and the two parallel surfaces are the two apolar/polar (interfacial) surfaces.

Figure 6.3

Computer simulation of TEM images. (A) Schematic illustration of TEM data in 2D projections of a specimen with a finite thickness. A 3D object (a) is depicted and is translucent to the projection rays of an electron beam; (b) representation of one unit cell of the gyroid surface; (c) projection plane onto which the rays impinge, in analogy of the film on which the image would be recorded; (d) 2D projection map provides a corresponding template for matching the patterned membrane domain in the TEM micrograph. (B) Comparison between a 3D cubic membrane model of a gyroid-based surface and its computer simulated projections at different viewing directions. Multiple 2D projections that are generated from the same 3D structure form a library of different patterns. The bottom row corresponds to computer-simulated projections for the top row, based on a projected specimen thickness of one-half of a unit cell viewed at the [1, 0, 0] (left), [1, 1, 0] (middle), and [1, 1, 1] (right) directions. The computer-generated projection can be matched with TEM micrographs to determine the 3D structure of a cubic membrane arrangement (see section 2.4. for further details).

Figure 6.4

Cell membrane organization. Schematic diagram depicting the likely 3D structure of annulated lamellae, tubulo-reticular structure (TRS) and the membrane folding transition. The pores of annulated lamellae may alternate in arrangement with the symmetry often being quadratic (A) or the pore face each other with the symmetry being hexagonal (B). Two examples of TRS membrane arrangements; (C) interconnected sacular (cisternae) and (D) tubular membrane organization show no global symmetry. A possible model of continuous membrane folding for the formation of double diamond (lower left) and gyroid (upper left) cubic type, hexagonal (upper right) and lamellar structures, and whorls (lower right) (E). The coexistence of these membrane organizations has been reported frequently in UT-1 and COS-7/CV-1 cells with HMG-CoA reductase and cytochrome b(5) overexpression, respectively. Panels A-D adapted from Figs. 17 and 18; Bouligand, 1991.

Figure 6.5

Examples of different membrane organizations observed in UT-1 cells, 48–72 h after compactin (40 μM) treatment (Deng et al., unpublished). (A) Annulate lamellae, (B) stacked undulated lamellae that show hexagonal transition, (C) cubic, and (D) hexagonal membrane morphologies may coexist in the same cell. Membrane folding appears to originate at the nuclear envelope or the endoplasmic reticulum.

Figure 6.6

Multilayer membrane organization and transformation. (A) An overview of the ultrastructure of chloroplast membrane in green algae Zygnema sp. (LB923) at 41 days of culture. Scale bar: 1 μm. (B) Several subdomains display different morphologies, ranging from simple stacked lamellar in direct association with paired parallel membranes (2 membranes; upper left) and double paired parallel membranes (4 membranes; lower right) of the gyroid-based cubic membrane morphology. Scale bar: 500 nm (Deng, 1998).

Figure 6.7

Direct template matching method. (A) TEM micrograph of lens mitochondria observed in the retinal cones of tree shrew species; (B) 6 pairs (12 layers) of G-based parallel level surfaces—a mathematical 3D model—that can be used to describe G type of cubic membrane morphology and the corresponding computer-simulated 2D projection map (C) derived from the corresponding 3D model in (B) (image provided by Prof. S. Wagon, St. Paul, Minnesota); TEM micrograph of lens mitochondria (A) perfectly match the theoretical projection (C), that is generated from 6 pairs (or 12 layers) of G-level surfaces (±0.1, ±0.2, ±0.4, ±0.5, ±0.7, ±0.8) with a quarter of a unit cell section thickness viewed from the lattice direction [1, 1, 1]. Note the matching details of the TEM projection and computer-simulated 2D projection such as the appearance of density of the lines (membranes) and the density between the sinusoid membranes. The original TEM micrograph in (A) is adopted from Fig. 6.10, from Foelix et al. (1997) with kind permission of Springer Science and Business Media. (14,000 ×).

Figure 6.8

Lipid dispersion prepared from amoeba lipids (Deng, unpublished). TEM images of liposomes derived from lipids extracted from fed and 7d starved Chaos cells. (A) Multilamellar or whorl-like structures generated from fed cell lipids with numerous randomly distributed tubular structures, but without higher order phases. In contrast, (B) TEM data of lipid dispersion generated from lipids that were isolated from 7d starved amoeba cells show highly ordered domains.

Figure 6.9

Chemical structures of three major lipids found in membrane lipids extracted from amoeba C. carolinense: plasmologen PC (16:0p/22:5), plasmologen PE (16:0p/22:5), and diacyl PI (22:5/22:5).

Figure 6.10

Electrostatic effects on cubic membrane organization (Deng, unpublished). Mitochondria of amoeba Chaos exhibiting cubic membrane arrangements were isolated in a buffer media containing (A) 50 μM, (B) 1 mM, or (C) 10 mM EDTA. Increasing the concentration of EDTA stabilized mitochondria with cubic morphology, suggesting a modulatory function of divalent cations in cubic membrane formation.

Figure 6.11

Cubic membrane organization and DNA uptake (Almsherqi et al., 2008). (A) Low and (B) high magnification TEM images of mitochondria containing cubic membrane structure isolated from 10 d starved Chaos cells before (A and B) and after (C and D) incubation with ODNs. Multiple electron-dense intra-mitochondrial inclusions (D) may represent cubic membrane-mediated ODN interactions. The multiple pores (B) at the surface of mitochondria with cubic membrane organization may play an important role in facilitating passive uptake of ODNs.