Optical tweezers across scales in cell biology

Abstract

Optical tweezers (OT) provide a noninvasive approach for delivering minute physical forces to targeted objects. Controlling such forces in living cells or in vitro preparations allows for the measurement and manipulation of numerous processes relevant to the form and function of cells. As such, OT have made important contributions to our understanding of the structures of proteins and nucleic acids, the interactions that occur between microscopic structures within cells, the choreography of complex processes such as mitosis, and the ways in which cells interact with each other. In this review, we highlight recent contributions made to the field of cell biology using OT and provide basic descriptions of the physics, the methods, and the equipment that made these studies possible.

Keywords: cytoskeleton; metastasis; microscopy; mitosis; motor proteins; optical trapping.

Figure 1:. Photons at work.

(A) Schematic of a photon (white comet) being scattered as it travels through a transparent particle (blue sphere). As the photon enters and exits the particle, its direction and momentum change. The difference between its original momentum and its altered momentum is reciprocally transferred to the particle, resulting in a force F (orange arrow). (B) Schematic of a focused laser beam, comprising numerous photons, applied to a transparent particle at equilibrium in the center of the trap and (C) a transparent particle displaced from the center of the trap and undergoing a force drawing the particle towards the trap’s center. (D) Schematic of a folded protein attached to a trappable particle. The particle is centered in the trap in (i), so there is no net force placed on it by the beam. With increasing OT forces (ii-iii), the protein is gradually unfolded. (E) Schematic of two optical traps holding (i) and stretching (ii) a red blood cell. The careful measurement of OT forces (See Box 1) yields information about the protein’s secondary and tertiary structures (D) and the cell’s viscoelasticity (E).

Figure 2:. Insights on mitosis, made possible by OT.

(A) A schematic cell that, for the purposes of illustration, shows different stages of mitosis (early to late, top to bottom). Letters and boxes refer to processes highlighted in panels (B-D). (B) A schematic representation of typical of OT experiments on motor proteins. In this case a MT is tethered to a bead, and OT of the bead measures the force generation or braking strength of the motor proteins tethering the trapped MT to another MT. This preparation is typical of recent studies in which teams of numerous individual motor proteins are studies working on their natural substrates, such as MTs [–56]. (C) An illustration of the in vitro preparation for studying ultra-fine bridge (UFB) recognition complexes (ref [65]). Pairs of beads allowed for OT tension to be applied to the synthetic UFB linking them. Various protein complexes (small circles) were tested for their effects on the synthetic UFBs’ stability and strength. (D) Laser cutting of one chromatid, which produces a fragment tethered near the telomere of the sister chromatid (right, tether not shown), leads to aneuploidy if the fragment is pulled in a cross-polar direction into the wrong daughter cell. This preparation serves as a model for naturally occurring aneuploidy and for the roles that tethers play in chromosome segregation and mis-segregation. OT can be used to pull the fragment away from its sister chromatid, thus allowing measurements of the strength and progressive dissolution of tethers as mitosis proceeds [64]. (E) With the NDC80 complex (green attachment) bound to an anchored MT and a trappable bead, OT was used to measure the forces harnessed by NDC80 as the MT depolarized, helping to explain how this kinetochore protein complex assists in transporting chromatids to the spindle poles during mitosis [68].

Figure I:. Common elements constituting optical tweezer setups.

(A). a simple OT microscope composed of trapping optics (along the red beam) and the imaging optics (along the blue beam). C: Camera, La: Laser, L: Lens, M: Mirror, DM: Dichroic Mirror, O: Objective, S: Sample, LS: Light source. (B). OT microscope composed of trapping optics and force measurement optics (along the red beam) and the imaging optics (along the blue beam). PD: Position Detector. (C). OT microscope composed of highly flexible trapping optics and force measurement optics (along the red beam) and the imaging optics (along the blue beam). SLM: Spatial Light Modulator.