Membrane skeleton

Editor-In-Chief: Henry A. Hoff

Overview
The interface formed by the interactions between the cell membrane and the cytoskeleton, often called the membrane skeleton (MSK), performs a variety of functions for the cell. The MSK works as a part of the plasma membrane as well as a part of the cytoskeleton. It is a scaffolding for membrane proteins to anchor to, and for forming organelles which extend from the cell. When high dynamicism is needed for drastic change in cell shape, effort is focussed at the MSK. It is involved in the localization of transmembrane proteins at specific sites in the cell membrane and in endocytosis and exocytosis in various cell types. The MSK provides the plasma membrane with the mechanical strength and resilience to withstand the stress and shear forces from the outside environment, while contributing stretch and shear elasticity. Signals originating at the cell membrane are transmitted to the cytoskeleton. It even participates in meiosis.

Introduction
An interface formed by the interactions between the plasma membrane and the cytoskeleton is often referred to as the plasma membrane skeleton. Co-ordinating the rearrangements of the actin portion of the cytoskeleton depends on its tight connection to the plasma membrane. There is a physical linkage of the cell membrane to the underlying MSK. The MSK may cover the entire cytoplasmic suface and is closely linked to clathrin-coated pits and caveolae. The MSK may differ from the bulk cytoskeleton in terms of its structure and protein composition, for its interactions with the plasma membrane in general and with specific molecules in the plasma membrane, and because it plays important roles in a variety of membrane functions.

Partitioning of the cell membrane
The MSK is involved in the localization of transmembrane proteins at specific sites in the cell membrane. A part of the MSK, directly and closely associated with the cytoplasmic surface of the plasma membrane, induces partitioning of the cell membrane with regard to the translational diffusion of membrane molecules, based on high speed single-particle tracking data on membrane proteins and lipids. In the short-time regime, these membrane molecules are temporarily confined within the compartments delimited by the MSK mesh, and, in the long-time regime, they undergo macroscopic diffusion by hopping between these compartments (MSK fence model). In the fence model, as a result of the collision of the cytoplasmic domains of transmembrane proteins with the MSK, transmembrane proteins are temporarily confined in the MSK mesh.

Lipid molecules also undergo hop diffusion.

Specific proteins that link the membrane and actin filaments at their barbed ends, such as gelsolin and villin, and at their sides, such as ponticulin and ezrin/radixin/moesin family proteins, occur at the interface of the MSK. In addition to actin and actin-associated proteins, some other proteins may contribute to forming the MSK and membrane corrals, e.g., septins and agorin.

Receptor-mediated endocytosis
Receptor-mediated endocytosis depends on the interaction between dynamin, a GTPase, and PtdIns(4,5)P2 (PIP2). The local adhesion between the plasma membrane and the actin cytoskeleton, determined by measuring the energy necessary to separate the plasma membrane from the underlying actin cytoskeleton, can be influenced by changes in the levels of PIP2 at the plasma membrane. PIP2 localizes to actin-rich structures in highly dynamic regions of the cell membrane and its retention and clustering at the plasma membrane influences actin cytoskeleton dynamics. High concentrations of PIP2 are accompanied by extensive actin polymerisation.

PIP2 directly binds to profilin, an actin-monomer-sequestering protein, and this binding induces the release of G-actin from the profilin-actin complex. PIP2 is a regulatory mechanism on profilin, regulating the association of profilin with actin. Profilin can regulate polymerization by binding to the ends of the actin filaments rather than the monomers.

Arf6, an ADP-ribosylation factor, and Rac colocalize at the plasma membrane where Arf6 plays a role in clathrin-mediated endocytosis and Fc-dependent phagocytosis. Arf6 may regulate membrane traffic and the cytoskeletal rearrangements associated with cell spreading and membrane ruffling.

Spectrin-actin network
Spectrins are a family of extended molecules comprised of α and β subunits that assemble with actin to form a subplasmalemmal lattice required for mechanical support of plasma membranes.

MSK provides the plasma membrane with the mechanical strength and resilience to withstand the stress and shear forces from the outside environment, which is well established in the thick cortical actin layers in immune cells and in the spectrin–actin network in red blood cells. The erythrocyte membrane skeleton, which is localized exclusively on the cytoplasmic surface of the plasma membrane, is a network of proteins, mainly spectrins, actins, and band 4.1.

Spectrin lines the intracellular side of the plasma membrane of many cell types in pentagonal or hexagonal arrangements, forming a scaffolding and playing an important role in maintenance of membrane integrity and cytoskeletal structure. Spectrins are linked to the plasma membrane through ankyrins and interactions with several molecules including membrane lipids, the NMDA receptor, the α subunit of the epithelial sodium channel, and the Na+/H+ exchanger.

The spectrin-based MSK is also linked by ankyrins, a family of adaptor proteins, to a number of integral membrane proteins including the anion exchanger, the Na+/Ca2+ exchanger, the Na+/K+ ATPase, members of the L1 CAM family, and voltage-gated sodium channels.

Signaling
Signals originating at the plasma membrane may be transmitted by PIP2 to the underlying actin cytoskeleton. Signaling intermediates including focal adhesion molecules as vinculin and members of the ERM and WASP families of proteins interact with PIP2.

By being in physical contact some cells can form gap junctions that connect their cytoplasm to the cytoplasm of adjacent cells or engage in juxtacrine signalling. Cells can receive information from their environment through a class of proteins known as receptors.

Physical linkage
Physical linkage of the plasma membrane to the underlying membrane skeleton (specifically actin filaments) is via binding with ankyrin, protein 4.2, α-actinin, spectrin, and dystrophin. Binding also occurs with band 3, which appears to be to prevent membrane surface loss, and vinculin. Like vinculin, titin is influenced by PIP2 to link actin filaments to the plasma membrane.

Supervillin (SVIL) is tightly associated with both actin filaments and plasma membranes, suggesting a role as a high-affinity link between the actin cytoskeleton and the membrane. Its function may include recruitment of actin and other cytoskeletal proteins into specialized structures at the plasma membrane.

Focal adhesions
Adhesion sites can be molecularly heterogeneous structures that have diverse morphologies, molecular compositions and phosphotyrosine levels (e.g. focal contacts, fibrillar adhesions, focal complexes and podosomes).

Focal adhesions can be considered as sub-cellular macromolecules that mediate the regulatory effects (e.g. cell anchorage) of extracellular matrix (ECM) adhesion on cell behavior. Focal adhesions serve as the mechanical linkages to the ECM, and as a biochemical signalling hub to concentrate and direct numerous signaling proteins at sites of integrin binding and clustering.

The submembrane plaque of focal adhesions contain a meshwork of >50 anchor proteins that link the membrane to the actin cytoskeleton. Vinculin can bind to the lipid bilayer and F-actin in its open conformation. PIP2 induces the transition of vinculin from closed conformation to open.

Lysophosphatidic acid (LPA) activation of Rho induces the formation of stress fibres and focal adhesions.

Cell junctions
A cell junction is a structure consisting of protein complexes that provides contact between neighbouring cells, between a cell and the extracellular matrix, or provides a paracellular barrier and control of paracellular transport within a tissue of a multicellular organism.



Adherens junction
Besides serving as a linker protein between the main elements of the cytoskeleton: actin microfilaments, microtubules and intermediate filaments, plectin links the cytoskeleton to junctions found in the plasma membrane that structurally connect different cells. Plectin binds subplasma membrane skeleton proteins such as α spectrin and fodrin (SPTAN1 and SPTBN1). Monomeric α-catenin preferentially binds to the cadherin junction complex through β-catenin. Dimeric α-catenin preferentially binds to actin and suppresses Arp2/3 complex-mediated actin branching, thus acting as a molecular switch to regulate actin polymerization. Since the membrane associated actin is several fold less stable compared to components of the adherens junctional complex, the actin branching must be suppressed.

Desmosomes and hemidesmosomes
Linking the cytoskeleton to intercellular junctions enables the structural integrity of a tissue as a whole. Two such junctions, desmosomes and hemidesmosomes, link intermediate filament networks between cells by association with plectin. Plectin localizes to the desmosomes and can form bridges between the desmosome protein, desmoplakin and intermediate filaments. In hemidesmosomes plectin interacts with the integrin β4 subunits of the hemidesmosome plaque and functions in a clamp like manner to crosslink the intermediate filament, cytokeratin, to the junction.

Attachment plaques
The attachment plaque (a disk-like structure) of a desmosome consists of desmoplakin and plakoglobins that bind to the intracellular domain of cadherens. The dense tripartite plaque structures of the desmosome and the hemidesmosome are specialized for intermediate filament anchorage. In the desmosome, mirror image plaques sandwich a membrane core, whereas in the hemidesmosome a single plaque serves this function. Hemidesmosomes are attached through an integrin-based mechanism to the membrane.

Plakoglobin is a common component of adhesive junctions. Plakoglobin binds tightly to the cytoplasmic domains of desmocollin and desmogleins and may serve as a molecular link between the outer and inner portions of the desmosomal plaque.

Desmoplakin probably acts as a molecular linker to anchor intermediate filaments at the desmosome. Both plakoglobin and desmoplakin are phosphoproteins. Another plaque component, dystonin, like desmoplakin, anchors keratin-containing intermediate filaments to hemidesmosomes. COL17A1, another plaque protein, has a region of 36 amino acids at the amino terminus that is required for its polarization in the plasma membrane. The perimembrane noncollagenous domain of COL17A1 appears to be essential for interactions with other hemidesmosomal components. The N terminus of COL17A1 interacts with the N-terminal domain of dystonin. Isoform 1 of dystonin has a distinct N-terminal domain that is capable of binding actin, whereas its C terminus interacts with perpherin-type intermediate filaments.

Gap junctions
Connexins are plasma membrane proteins. As a hexamer they form a channel, a connexon. Two connexons in neighboring cells may dock to form a gap junction channel. Each connexon has its N- and C-terminal tails and one connecting loop within the cytoplasmic region. Gap-junctional plaque can contain up to three different types of connexin. Colocalization of actin filaments and connexin can occur and this colocalization is significantly increased when gap-junction cells are subjected to cyclic strain. Connexins may associate with actin microfilaments to stabilze gap junctions at the cell membrane cytoplasmic surface to ensure that the cells remain coupled during prolonged periods or intense mechanical loading. Debrin (an actin binding protein) is a binding partner of the connexin COOH-terminal domain and connexion interacts with the drebrin-containing submembrane cytoskeleton.

Tight junctions
Claudins and the occludins associate with different peripheral membrane proteins located on the intracellular side of the plasma membrane which anchor the strands to the actin cytoskeleton.

Tight junction protein 1 (zona occludens 1) (TJP1) GeneID: 7082, interacts with claudin 1-8, junctional adhesion molecules 2 & 3, occludin, TJP2 & 3, actin, alpha catenin, and cingulin.

Meiosis
Aster microtubules radiate from the centrosome into the cytoplasm. They are in contact with the plasma membrane and components of the membrane skeleton.

Cell membrane extensions
There are a variety of local convex outward extensions or outgrowths of the cell surface. Each of these is sustained by a corresponding internal extension of the membrane skeleton.



Lamellipodia
The cell surface extends a membrane process, termed a lamellipodium in pseudopod formation. The lamellipodium is born of actin nucleation in the plasma membrane and is the primary area of actin incorporation or microfilament formation of the cell. Polymerization of actin takes place to form filaments at the leading edge, which subsequently blend into one another to form networks (a two-dimensional actin mesh). The whole structure pulls the cell across a substrate. Actin polymerization may be at the origin of the force propelling the membrane forwards. Lamellipodia are a characteristic feature at the front, leading edge, of motile cells.

Local microtubule fragmentation in the presence of katanin occurs in lamellipodia during developmental migration of interphase. Ruffling and protrusion of the leading edge is dependent on directed F-actin assembly nucleated at or near the inner plasma membrane surface along the leading edge. Microtubule dynamic instability is required for cell migration. Proper arrangement of microtubules is apparently needed during cell migration and this may be maintained by F-actin retrograde flow. Microtubule dynamic instability and actin-based retrograde flow may be required to work together to model and remodel the microtubule cytoskeleton for continued movement.

Filopodia
Within lamellipodia are ribs of actin referred to as microspikes, which, when they spread beyond the lamellipodium frontier, are called filopodia. Drebrin, a dendritic spine-resident side-binding protein of filamentous actin (F-actin), is responsible for recruiting F-actin and DLG4 into filopodia. Actin microfilaments and dispersed myosin fill filopodia.

Filopodia may be precursors of dendritic spines.

Axonal growth cones
An axonal growth cone is a highly motile structure at the growing tip of an axon. Growth cones contain receptors and apparently when a growth cone senses a guidance cue, the receptors activate various signaling molecules in the growth cone that eventually affect the cytoskeleton.

NRCAM, an ankyrin-binding protein, is involved in neuron-neuron adhesion and promotes directional signaling during axonal cone growth. It is also expressed in non-neural tissues and may play a general role in cell-cell communication via signaling from its intracellular domain to the actin cytoskeleton during directional cell migration.

Integrins expressed on axonal cone membranes mediate the adherence of neurons to the glycoproteins of the extracellular matrix (ECM).

Katanin may sever centrosomal microtubules in certain types of interphase cells such as epithelial cells that must deploy microtubules through the cytoplasm. With regulated katanin present axonal growth cones pause, and microtubules fragment, at sites of branching during neural development. There is breakage of microtubules at the axonal branch points and in the growth cones of the neurons. Microtubules move independently in both anterograde and retrograde directions within axonal growth cones and developing interstitial branches and act as railways for the transport of various materials. Average velocities for mobile microtubules in axonal growth cones ranged from 4.2 to 8.6 µm/min, with peak velocities from 7.2 to 21.8 µm/min.

In neuronal growth cones, the rearward movement of F-actin is myosin dependent. Microtubule dynamic instability is required for cell migration. The F-actin-based retrograde flow may be needed for proper arrangement of microtubules during cell migration. Inhibition of actin polymerization at the leading edge may allow F-actin and microtubules to be drawn rearward as a composite cytomatrix by myosin.

Axonal initial segments
An axonal initial segment (AIS) is a specialized domain that maintains the polarized distribution of membrane proteins in a neuron. It is the initiation site of action potentials and is characterized by a dense undercoating and bundles of microtubules connected by cross-bridges.

Ankyrin-G coordinates the physiological assembly of transmembrane adhesion molecules, voltage-gated sodium channels, and the spectrin membrane skeleton into a protein complex at axon initial segments. The spectrin membrane skeleton includes betaIV spectrin.

Amphophysin II (BIN 1) is associated with the cytoplasmic surface of synaptic vesicles, and it is concentrated in the cortical cytomatrix of axon initial segments. Amphiphysin II is colocalized with splice variants of ankyrin3 (ankyrinG), a component of the actin cytomatrix. Amphiphysin family members may have a role both in endocytosis and in actin function.

Dendritic spines
Debrin, which connects gap junctions to the actin portion of the membrane skeleton, has a well-established role in the morphogenesis, patterning and maintenance of dendritic spines in neurons. Debrin may be broadly involved in shaping cell processes and in the formation of stabilized plasma membrane domains.

Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.

Initial content for this page in some instances came from Wikipedia.