Cytoskeletal mechanisms for breaking cellular symmetry

Abstract

Cytoskeletal systems are networks of polymers found in all eukaryotic and many prokaryotic cells. Their purpose is to transmit and integrate information across cellular dimensions and help turn a disorderly mob of macromolecules into a spatially organized, living cell. Information, in this context, includes physical and chemical properties relevant to cellular physiology, including: the number and activity of macromolecules, cell shape, and mechanical force. Most animal cells are 10-50 microns in diameter, whereas the macromolecules that comprise them are 10,000-fold smaller (2-20 nm). To establish long-range order over cellular length scales, individual molecules must, therefore, self-assemble into larger polymers, with lengths (0.1-20 m) comparable to the size of a cell. These polymers must then be cross-linked into organized networks that fill the cytoplasm. Such cell-spanning polymer networks enable different parts of the cytoplasm to communicate directly with each other, either by transmitting forces or by carrying cargo from one spot to another.

Figure 1.

Mechanisms by which cytoskeletal networks integrate information over long distances. (A) Pushing forces generated by polymer assembly or network expansion, (B) pulling forces caused by motor sliding filaments, and (C) oriented tracks for delivery of cargo. Note that pulling forces can also be generated by polymer disassembly (Lombillo et al. 1995) and pushing forces can be generated by motor-mediated filament sliding, but these processes are less common.

Figure 2.

Positive feedback in the activation of the Arp2/3 complex. (A) Molecular mechanism of Arp2/3 activation. (a) Four factors must come together to produce a new filament: An Arp2/3 complex, an actin monomer, a VCA domain, and a pre-existing actin filament. (b) All the components assemble into a preactivation complex. (c) A conformational change in the complex results in formation of a new stable filament (d) attached to the side of the pre-existing (mother) filament. (B) The requirement for a pre-existing filament makes the nucleation reaction auto-catalytic. The existence of a single filament near a surface covered with an Arp2/3 activator leads to nucleation of more filaments, which, in turn, stimulate further Arp2/3-dependent nucleation.

Figure 3.

Typical heterogeneities in the architecture of polymer networks. (A) Variations in polymer density. Arrow denotes region of low density. Lower panel: Electron micrograph of actin filaments in a sea urchin coelomocyte (Henson et al. 1999). (B) Variations in orientation or alignment of polymers. Arrow denotes region of filament alignment in an otherwise random gel. Lower panel: Variations in actin filament alignment in the periphery of a fibroblast cell (Svitkina et al. 2003). (C) Variations in density or activity of accessory factors such as motor or cross-linker molecules. Arrow denotes region of high concentration of motor or cross-linker. Lower panel: distribution of myosin motors (yellow) in the actin network of a fish epidermal keratocyte (Svitkina and Borisy 1997).

Figure 4.

Elastic versus viscous behavior of cross-linked polymer networks. (A) Some cross-linked cytoskeletal networks respond to some types of applied forces (arrows) like elastic materials. They deform and then return to their original shape after the force is removed. (B) Other networks flow like viscous liquids in response to applied forces and do not return to their original shape after the force is removed. For most networks, the response to applied forces is best described by a combination of these viscous and elastic behaviors.

Figure 5.

Assembly of polarized, radial arrays of microtubules. (A) Multi-valent microtubule-based motors can cross-link microtubules and cause them to slide past each other. (B) The polarity of the motors (in this case, minus end-directed) causes the minus ends of the microtubules to be pulled into proximity. (C) Additional motor-dependent interactions recruit additional microtubules, eventually forming (D) a polarized radial array.

Figure 6.

Formation of bipolar spindles in the absence of centrosomes. (A) In cell extracts, DNA-coated particles induce nucleation of microtubules whose polarities are random with respect to the DNA. (B) Microtubule cross-linkers and multivalent motors align the microtubules while minus-end directed motors begin to cluster their minus ends. (C) Plus end-directed microtubule motors associated with DNA push the clustered minus ends away from the DNA-coated particles, forming a bipolar, spindle-shaped structure.

Figure 7.

Polarization of actin networks because of expansion and mechanical rupture. (A) Spherical surfaces coated with activators of the Arp2/3 complex (black circle) direct assembly of spherically symmetrical actin networks (gray rings). Continuous nucleation of new filaments at the surface causes outward displacement and stretching of the initial actin shell (1). (B) Continued expansion eventually stretches the outer portions of the actin network to the point of mechanical failure. The elastic energy stored in the stretched shells is released by contraction. The initial rip propagates because of positive feedback. As the rip progresses, the remaining stress in the network is distributed over a smaller number of cross-links, increasing the probability that that they will fail. (C) Recoil of the actin shell away from the rip results in a polarized distribution of actin on the surface of the particle.

Figure 8.

Polarization and motility of cytoplasts. (A) In cytoplasts that have lost polarization, actin assembly occurs symmetrically around the periphery, producing uniform outward forces (arrows) that sum to zero. Myosin II minifilaments (red symbols) are randomly oriented and do not produce coherent, large-scale pulling forces. (B) Random fluctuations in myosin localization or activity or external mechanical perturbation (large arrow) can produce local alignment of actin filaments. (C) Alignment of filaments produces more favorable binding sites for myosin minifilaments and makes their pulling forces more coherent (arrows at bottom of cell). As the contraction proceeds, more filaments are aligned and recruited into the contractile bundle. This bundle is a poor substrate for the Arp2/3 complex and inhibits production of polymerization-driven outward forces. This imbalance of outward forces enables polymerization on the opposite side of the cell to produce net protrusion. (D) Translocation of the cell in the direction of protrusion results in treadmilling of the actin network away from the leading edge. This treadmilling carries myosin II away from the leading edge and concentrates it in the rear of the cell, further enhancing the polarization of the cell and the spatial separation of protrusive and contractile forces.

Figure 9.

Contraction-induced polarization of C. elegans zygotes. (A) Before polarization, many posterior determinants (e.g., par6, yellow symbols) are uniformly distributed throughout the cortical actin network of the zygote. Soon after sperm entry, the activity of myosin II minifilaments (red symbols) causes contraction of the cortical actin network. The sperm centrosome (near the right side of the zygote) delivers factors to the cortex that locally inhibit myosin activity. (B) The asymmetric activity of myosin II pulls the cortical actin network away from the sperm centrosome toward the opposite end of the cell. (C) Actin-associated polarity determinants pulled along with the cortical network define the posterior pole of the zygote and, ultimately, the embryo. Cortical actin removed from the future anterior pole is replaced by newly polymerized filaments or filaments that well up to the surface from the underlying cytoplasm (green filaments).