Reconstitution of Cortical Dynein Function

In Vitro Reconstitution of Dynamic Co-organization of Microtubules and Actin Filaments in Emulsion Droplets

In vitro (or cell-free) reconstitution is a powerful tool to study the physical basis of cytoskeletal organization in eukaryotic cells. Cytoskeletal reconstitution studies have mostly been done for individual cytoskeleton systems in unconfined 3D or quasi-2D geometries, which lack complexity relative to a cellular environment. To increase the level of complexity, we present a method to study co-organization of two cytoskeletal components, namely microtubules and actin filaments, confined in cell-sized water-in-oil emulsion droplets. We show that centrosome-nucleated dynamic microtubules can be made to interact with actin filaments through a tip-tracking complex consisting of microtubule end-binding proteins and an actin-microtubule cytolinker. In addition to the protocols themselves, we discuss the optimization steps required in order to build these more complex in vitro model systems of cytoskeletal interactions.

Microtubule-dependent pushing forces contribute to long-distance aster movement and centration in Xenopus laevis egg extracts

During interphase of the eukaryotic cell cycle, the microtubule (MT) cytoskeleton serves as both a supportive scaffold for organelles and an arborized system of tracks for intracellular transport. At the onset of mitosis, the position of the astral MT network, specifically its center, determines the eventual location of the spindle apparatus and ultimately the cytokinetic furrow. Positioning of the MT aster often results in its movement to the center of a cell, even in large blastomeres hundreds of microns in diameter. This translocation requires positioning forces, yet how these forces are generated and then integrated within cells of various sizes and geometries remains an open question. Here we describe a method that combines microfluidics, hydrogels, and Xenopus laevis egg extract to investigate the mechanics of aster movement and centration. We determined that asters were able to find the center of artificial channels and annular cylinders, even when cytoplasmic dynein-dependent pulling mechanisms were inhibited. Characterization of aster movement away from V-shaped hydrogel barriers provided additional evidence for a MT-based pushing mechanism. Importantly, the distance over which this mechanism seemed to operate was longer than that predicted by radial aster growth models, agreeing with recent models of a more complex MT network architecture within the aster.

Minimal in vitro systems shed light on cell polarity

Cell polarity - the morphological and functional differentiation of cellular compartments in a directional manner - is required for processes such as orientation of cell division, directed cellular growth and motility. How the interplay of components within the complexity of a cell leads to cell polarity is still heavily debated. In this Review, we focus on one specific aspect of cell polarity: the non-uniform accumulation of proteins on the cell membrane. In cells, this is achieved through reaction-diffusion and/or cytoskeleton-based mechanisms. In reaction-diffusion systems, components are transformed into each other by chemical reactions and are moving through space by diffusion. In cytoskeleton-based processes, cellular components (i.e. proteins) are actively transported by microtubules (MTs) and actin filaments to specific locations in the cell. We examine how minimal systems - in vitro reconstitutions of a particular cellular function with a minimal number of components - are designed, how they contribute to our understanding of cell polarity (i.e. protein accumulation), and how they complement in vivo investigations. We start by discussing the Min protein system from Escherichia coli, which represents a reaction-diffusion system with a well-established minimal system. This is followed by a discussion of MT-based directed transport for cell polarity markers as an example of a cytoskeleton-based mechanism. To conclude, we discuss, as an example, the interplay of reaction-diffusion and cytoskeleton-based mechanisms during polarity establishment in budding yeast.

Use of Xenopus cell-free extracts to study size regulation of subcellular structures

Striking size variations are prominent throughout biology, at the organismal, cellular, and subcellular levels. Important fundamental questions concern organelle size regulation and how organelle size is regulated relative to cell size, also known as scaling. Uncovering mechanisms of organelle size regulation will inform the functional significance of size as well as the implications of misregulated size, for instance in the case of nuclear enlargement in cancer. Xenopus egg and embryo extracts are powerful cell-free systems that have been utilized extensively for mechanistic and functional studies of various organelles and subcellular structures. The open biochemical nature of the extract permits facile manipulation of its composition, and in recent years extract approaches have illuminated mechanisms of organelle size regulation. This review largely focuses on in vitro Xenopus studies that have identified regulators of nuclear and spindle size. We also discuss potential relationships between size scaling of the nucleus and spindle, size regulation of other subcellular structures, and extract experiments that have clarified developmental timing mechanisms. We conclude by offering some future prospects, notably the integration of Xenopus extract with microfluidic technology.

Optical Tweezers-Based Measurements of Forces and Dynamics at Microtubule Ends

Microtubules are dynamic cytoskeletal polymers that polymerize and depolymerize while interacting with different proteins and structures within the cell. The highly regulated dynamic properties as well as the pushing and pulling forces generated by dynamic microtubule ends play important roles in processes such as in cell division. For instance, microtubule end-binding proteins are known to affect dramatically the dynamic properties of microtubules, and cortical dyneins are known to mediate pulling forces on microtubule ends. We discuss in this chapter our efforts to reconstitute these systems in vitro and mimic their interactions with structures within the cell using micro-fabricated barriers. Using an optical tweezers setup, we investigate the dynamics and forces of microtubules growing against functionalized barriers in the absence and presence of end-binding proteins and barrier-attached motor proteins. This setup allows high-speed as well as nanometer and piconewton resolution measurements on dynamic microtubules.

Understanding force-generating microtubule systems through in vitro reconstitution

Microtubules switch between growing and shrinking states, a feature known as dynamic instability. The biochemical parameters underlying dynamic instability are modulated by a wide variety of microtubule-associated proteins that enable the strict control of microtubule dynamics in cells. The forces generated by controlled growth and shrinkage of microtubules drive a large range of processes, including organelle positioning, mitotic spindle assembly, and chromosome segregation. In the past decade, our understanding of microtubule dynamics and microtubule force generation has progressed significantly. Here, we review the microtubule-intrinsic process of dynamic instability, the effect of external factors on this process, and how the resulting forces act on various biological systems. Recently, reconstitution-based approaches have strongly benefited from extensive biochemical and biophysical characterization of individual components that are involved in regulating or transmitting microtubule-driven forces. We will focus on the current state of reconstituting increasingly complex biological systems and provide new directions for future developments.

Reconstitution of Basic Mitotic Spindles in Spherical Emulsion Droplets

Mitotic spindle assembly, positioning and orientation depend on the combined forces generated by microtubule dynamics, microtubule motor proteins and cross-linkers. Growing microtubules can generate pushing forces, while depolymerizing microtubules can convert the energy from microtubule shrinkage into pulling forces, when attached, for example, to cortical dynein or chromosomes. In addition, motor proteins and diffusible cross-linkers within the spindle contribute to spindle architecture by connecting and sliding anti-parallel microtubules. In vivo, it has proven difficult to unravel the relative contribution of individual players to the overall balance of forces. Here we present the methods that we recently developed in our efforts to reconstitute basic mitotic spindles bottom-up in vitro. Using microfluidic techniques, centrosomes and tubulin are encapsulated in water-in-oil emulsion droplets, leading to the formation of geometrically confined (double) microtubule asters. By additionally introducing cortically anchored dynein, plus-end directed microtubule motors and diffusible cross-linkers, this system is used to reconstitute spindle-like structures. The methods presented here provide a starting point for reconstitution of more complete mitotic spindles, allowing for a detailed study of the contribution of each individual component, and for obtaining an integrated quantitative view of the force-balance within the mitotic spindle.

Centrosomal clustering contributes to chromosomal instability and cancer

Cells assemble mitotic spindles during each round of division to insure accurate segregation of their duplicated genome. In animal cells, stereotypical spindles have two poles, each containing one centrosome, from which microtubules are nucleated. By contrast, many cancer cells often contain more than two centrosomes and form transient multipolar spindle structures with more than two poles. In order to divide and produce viable progeny, the multipolar spindle intermediate must be reshaped into a pseudo-bipolar structure via a process called centrosomal clustering. Pseudo-bipolar spindles appear to function normally during mitosis, but they occasionally give rise to aneuploid and transformed daughter cells. Agents that inhibit centrosomal clustering might therefore work as a potential cancer therapy, specifically targeting mitosis in supernumerary centrosome-containing cells.